1. Field of the Invention
The present invention relates generally to toner used in toner cartridges, and in particular, to techniques for improving the fusibility of a magenta color toner.
2. Description of the Related Art
Instant-on fusers have become an essential element of the competitive color printer, nearly independent of the market niche or printer speed. Due to their low thermal mass and compact size, belt fusers may comprise one way to achieve this feature in a small-footprint printer. The fuser may be comprised of a flexible belt with a low thermal mass which passes over the surface of a heating element. There may be a back up member of some design which permits the heated surface to contact the powder image to be fused. Such a thermal belt fuser may be quite different from the extremes of thermal fusing methods, in that the shear rate applied, or pressure involved, in fusing may be minimal.
In the case of radiant fusing at one extreme, there may be no contact between the heat source and an unfused powder image, and as such there may be essentially a zero shear rate involved, which means that in order for the toner to flow and melt and adhere to the substrate, it must be heated sufficiently such that the viscosity may be reduced to a level to permit irreversible viscous flow. The other fusing extreme may involve the heating of a powder image while it is simultaneously subjected to pressure, and thus the irreversible viscous flow of the polymer melt may be possible at a much lower temperature than in the absence of pressure. At high pressures, polymer flow may be readily achieved at much lower temperatures than if no pressure is applied.
The belt fuser is a case where there is a finite pressure applied during the fusing event, but a minimal amount. Thus, the temperature must be high enough to reduce polymer viscosity at low applied pressures, or low shear rates. This polymer property associated with toner fusing under conditions of low shear may be referred to as ‘low shear viscosity’ or ‘Non-Newtonian’ viscosity. At such low shear rates, polymer chain entanglement may inhibit free viscous flow. As shear rate or applied pressure increases, polymer viscosity may decrease and fusing may be facilitated at any given temperature by the higher shear rate.
Toners for electrophotographic printers may be made by a conventional process involving a melt mix of resin(s), wax(es), pigment(s), and other additives. This material may then be subjected to a grinding process, which produces toner particles of roughly 10 microns. Smaller sizes can be achieved through this method, however limitations exist. The chemically produced toner (CPT) process may be performed by emulsion aggregation, suspension, or chemical milling. Chemically producing toner allows a smaller particle size toner to be produced that has tighter control of the particle shape and the particle size distribution. Fusing of a toner powder essentially comprises the melting and viscous flow of a filled polymer or filled polymer blend.
Toners are mainly made of binder (polymeric) resins and coloring agents, to which other materials are added including waxes that improve low-temperature fusing property onto a recording sheet and releasing property from the fusing member, charge control agents that add polarity (positive or negative electric charge), etc. Mainstream toner binder resins include styrene-acrylate resins and polyester resins.
The current state of the fusibility of toners is a process whereby both toner properties and fuser design may have to be modified to attain the desired performance of the toner on the substrate. Of the four colors, cyan, yellow, black and magenta, magenta has traditionally been relatively difficult color to fuse and to release from the fusing member. For that reason, 100% magenta image targets have been used as the fuser benchmark for determining acceptable performance. If the magenta solid image is considered to fuse, all other colors and combinations thereof will fuse. Few images are composed of 100% magenta toner, so perhaps it could be argued that this is too severe a test. Perhaps equally important, magenta is generally the uppermost (top) color in the layered color image. This means that upon fusing, magenta may be the color which contacts and releases from the fusing member, and may be the more difficult to fuse.
Improvements in the fusibility of magenta toner may therefore translate into improvements in the fusibility of full color imagery in electrophotographic devices.